1. Field of the Invention
This invention relates to a method of manufacturing a solar cell of compound semiconductors by way of coating and firing.
2. Description of the Prior Art
In recent years, expectations for solar cells as a clean energy source have been raised, in view of the global warming, acid rain, ozone layer destruction, and other such environmental destruction. For the wide usage of solar cells to occur, improvement of the photo-electric conversion efficiency and reduction of the cost are most important. that purpose, solar cells made of compound semiconductors of Group III-V materials such as GaAs, InP, Group II-VI materials such as CdS/Cu.sub.2 S, CdS/CdTe, and Group I-III-VI.sub.2 materials such as CuInS.sub.2, CuInSe.sub.2, as well as crystalline and amorphous silicon solar cells, have been investigated in many countries of the world. Among these, solar cells made of compound semiconductor heterojunctions of n-CdS/p-CdTe have been produced commercially, with relatively low material cost, conversion efficiency as high as 10%, less deterioration over long time periods, and a relatively simple manufacturing process suitable for mass production consisting of printing, drying, firing (sintering or baking), resulting in a high density arrangement on a glass plate and realization of high voltage without outer wire connection, as well as large area cells.
A typical solar cell of Group II-VI semiconductor, of which a sectional view is shown in FIG. 1, comprises a glass substrate 1 of high light transmittance and electrical insulation provided on one surface thereof with an n-type CdS layer 2, a p-type CdTe layer 3, a current collecting carbon electrode layer 4, an Ag.In electrode which is the positive terminal 5, and an Ag.In electrode which is the negative terminal 6 formed by laminating with printing and baking of each layer. Usually, although not shown in the figure, the thus prepared solar cell element is provided, on both the Ag-In electrodes, with a copper paste layer deposited, dried, and baked for easy soldering of lead wires. The cell is then covered all over with a passivation layer of a thermosetting resin such as epoxy and baked.
Light, including that of the sun, falls on the surface of the glass substrate 1 opposite to the surface having the above solar cell element layers, to generate electrical power by photo-electric conversion.
As the substrate, a heat-resistant barium borosilicate glass is employed, which has very low alkali metal content and a high softening point.
In the manufacturing of the compound semiconductor solar cell by the coating and firing method, it is important that each of the n-type compound semiconductor layer, p-type compound semiconductor layer, and electrode layer have uniform thickness, a smooth surface, and no pin-holes. Especially, if the n-type CdS semiconductor layer formed directly on the substrate is uniform, smooth, and non-porous, the adherence of the layer to the substrate is improved, resulting in an increase of the light transmittance, decrease of the sheet resistance, and, further, an increase of the photo-current and improvement of the characteristics of the cell.
Conventionally, to obtain such a layer, a paste made of the powdered compound semiconductor or elements therefor, an eletroconducting agent, and a viscous agent mixed together was kept under reduced pressure to remove bubbles therein and, after the deposition, the substrate was held horizontally at about 50.degree. C., which was lower than the drying temperature of the viscous agent, to reduce the viscosity of the viscous agent and uniformly precipitate the raw material powders in order to obtain a high density layer. However, if the bubbles were removed from the paste before coating, it sometimes happened in the coating process by screen printing that bubbles were introduced from the surrounding atmosphere, resulting in uneven deposition or pin-holes. Also, with the heat treatment only after coating, the raw material powders did not always uniformly precipitate, and the bubbles were not sufficiently removed, resulting in the layer not being flat, or of uniform thickness. The pin-holes left after coating and firing of the layers caused an increase of the sheet resistance. Especially, if pin-holes were formed in the p-type CdTe layer, the carbon particles of the carbon electrode layer formed thereon penetrated into the pin-holes up to the CdS layer under the CdTe layer, causing internal short circuiting or current leakage, fatally damaging the solar cell performance.